A common requirement in molding and casting processes is to measure the flow of heat from the molded or cast object, through the body of the mold to a liquid coolant or to the outside air. This is a difficult measurement to make, because:
1. most molding and casting processes involve very high temperatures;
2. the casting or molding process environment is extremely dirty, often electrically noisy;
3. molds are typically made of solid metal with high thermal conductivity; and
4. typical commercially available heat flux sensors have low thermal conductivity.
U.S. Pat. No. 5,360,051, issued to Takahashi et al, describes a typical requirement for heat flux measurements in a continuous casting process. The solution offered by the patentee is to embed thermocouples in the wall of the mold. Heat flux through the mold may be calculated from the signals of these thermocouples, which indicate temperatures of the mold body at various points. Heat flux is calculated using measured or assumed values of the mold""s thermal properties.
An alternative to the thermocouples of Takahashi et al would be a plug-type heat flux sensor as described by Liebert et al in U.S. Pat. Nos. 5,048,973, 5,086,204 and 5,314,247. The sensor described in these patents is an isolated plug directly machined into the wall of a vessel, optimally by electro-discharge machining, with thermocouples placed at various depths on the outer surface of the isolated plug. Heat flux is calculated from temperature data derived from the thermocouples, using a temperature variant thermal property inverse heat conductive problem method. These calculations of heat flux are extremely susceptible to electrical noise in the thermocouple signals. Any error in locating the thermocouples on the plug surface translates directly into an error in the heat flux calculation. The insulating gap between the instrumented plug and the surrounding material allows the plug temperature profile to depart from that in the surrounding material, depending on conditions at the open end of the plug. This can produce large errors.
An alternative to these methods would be to apply heat flux sensors to the surface of the mold. Such sensors are described in U.S. Pat. No. 4,567,365 issued to Degenne, U.S. Pat. No. 5,990,412 issued to Terrell, and U.S. Pat. No. 6,278,051, issued to Peabody. Heat flux sensors based on the teaching of these patents are commercially available, but they are not suitable for measurements in molding and casting processes. Their attachment to the outer surface of a mold adds a large local thermal resistance which causes heat to be shunted around the area covered by the sensor. The resulting measurements may be inaccurate as well as sensitive to local air currents and other conditions, and the sensors themselves are vulnerable to damage.
Ideally the flow of heat in a casting or molding process would be measured by a thermopile-type heat flux sensor imbedded in the mold itself. However, if the thermal conductivity of such a sensor were greatly different from that of the surrounding material, the pattern of heat flow through the mold would be distorted in the region of the sensor. This would produce a systematic error in the heat flux measurement. Thus it would be important for the sensor""s thermal conductivity to nearly match that of the mold. Also, voids or air spaces could not be introduced into the mold when the sensor is installed, because these would produce even more serious distortions of the heat flow in the region of the sensor.
The conventional way to achieve good noise immunity for a thermopile-type heat flux sensor is to raise its output voltage by increasing the temperature drop it introduces into the heat flow path. While this approach is acceptable in radiative heat flux measurements, it cannot be used for conductive heat flux measurements because of the large error it produces. When the thermal conductivity of an imbedded sensor is made approximately equal to that of the mold, the only ways to increase the heat flux signal are by increasing the number of thermocouple pairs or by increasing their physical separation in the direction of heat flow. Space is not often available for the large sensor that would be required.
U.S. Pat. No. 4,779,994, issued to Diller et al, teaches the application of a thin film thermopile heat flux sensor to a surface, for measurement of convective or radiative heat flux through the surface. The output voltage of these sensors is small despite their construction with hundreds of thermocouple pairs, because the thermal resistance they place in the path of heat flow is extremely small. Typically the resistive element consists of one micron (10xe2x88x926 meter) of a ceramic such as silicon monoxide. The thin films of these sensors are vulnerable to damage by abrasion and chemical attack, so they would not be suitable for the molding and casting environment.
A sensor designed for measurement of conducted heat flux passing through a solid object consists of a thin film thermopile deposited on a plane surface of a thin rectangular substrate. The thermopile is protected by being covered by a thin rectangular plate of the same material as the substrate. The sensor fits tightly in a slot in a threaded plug. For a measurement of heat flux in the solid object the threaded plug is imbedded in the solid object. Thermal properties of the substrate, the plate and the threaded plug match those of the solid object. When heat flows through the solid object the output voltage of the thermopile indicates the magnitude of the heat flux vector along the thermopile axis. Because the thermal properties of the substrate, plate and plug match those of the solid object, there is minimal deviation of the heat flow pattern from that which would have existed without the sensor present. Accurate and precise measurements of heat flux are the result. Applications include measurement of heat flux in casting molds, boiler tubes, well surveying instruments and laser weapons testing.